Leading explanation for ancient giant, flying insects gets squashed

Three hundred million years ago, the skies of the late Palaeozoic era were dominated by colossal insects like Meganeuropsis permiana, a formidable predator akin to today's dragonflies, boasting a wingspan exceeding 70 centimeters and weighing around 100 grams. For decades, scientists pondered why such massive insects no longer roam the Earth. A popular theory known as the "oxygen constraint hypothesis" suggested that these enormous creatures required highly oxygenated air to thrive, due to the inefficiencies of their respiratory systems compared to mammals and birds. The prevailing belief was that as atmospheric oxygen levels decreased, the survival of giant insects became untenable. However, recent insights challenge this long-held notion. Edward Snelling, a veterinary science professor at the University of Pretoria, argues that the explanation is overly simplistic and fundamentally flawed. Unlike mammals, insects do not possess centralized lungs or a closed circulatory system; instead, they breathe through a network of tubes called the tracheal system. Air enters their bodies through small openings on their exoskeleton known as spiracles, then travels through progressively smaller tubes called tracheae, branching into even tinier tracheoles that reach deep into their tissues. Insects have the ability to actively pump air into these larger tracheae by flexing their bodies, but the final delivery of oxygen to tissues depends on passive diffusion, which is inherently slow. The oxygen constraint hypothesis posited that the larger an insect grows, the more challenging it becomes for oxygen to reach its deepest tissues due to the increased distance. According to Snelling, this presents a significant issue: as insects increase in size, the demand for oxygen in their muscles escalates. To meet this demand, larger insects would require either significantly larger or more numerous tracheoles, suggesting a critical structural limit. If an insect grows too large, the necessary breathing tubes would occupy too much space, impeding their muscle function and ultimately compromising their flight capabilities.